The invention relates generally to semiconductor integrated circuit (IC) devices and more particularly, it relates to a local oscillator (LO) frequency generation circuit for use in a transmitter or a receiver of a communication device operating for example in a wireless local area network. The invention is particularly relevant for combo devices requiring a multi-band local oscillator such as an IEEE802.11g wireless device. Other applications for the invention comprise transmitters or receivers that need to operate at various frequency bands over a wireless or terrestrial medium.
Devices with local frequency oscillators have been used in many applications, ranging from cellular phones to television tuners to wireless local area networks (WLAN) or wireless personal area networks that implement various standardized protocols, e.g., IEEE802.15, IEEE 802.11 a/b, HiperLAN2 or Zigbee. A wireless device receives radio signals through its receiver via an antenna. The radio signals are mixed with LO frequency signals generated from a LO frequency generation circuit for further processing to generate baseband signals. The receiver is typically capable of receiving radio signals in multiple frequency bands. Currently, there are several ways to realize the LO frequency band switch for generating frequency signals to produce multiple frequency bands.
Conventionally, a simple implementation comprises several voltage-controlled oscillators (VCO), one for each of the frequencies desired to be generated and a multiplexer. The multiplexer allows selection of a given frequency from the desired frequencies. The main drawback of this technique is the long switching time and the large silicon area required.
Another approach is to use only one VCO in conjunction with a capacitor bank. The wanted frequency is then generated by switching among selected capacitors in the capacitor bank.
Another known technique is to use one VCO followed by a frequency divider with a divisor of N. By switching on or off the frequency divider, frequencies that are integer subharmonics of the VCO frequency can be generated. The limitation of this technique is that it works only for small integers and cannot generate non-integer band frequency ratios. Furthermore, a high frequency VCO is usually required.
Therefore, there is a need for an improved LO frequency generation circuit with superior performance.
It is an object of the invention to provide a multi-band local oscillator implementation with a unique oscillator generating the various frequency bands.
It is another object of the invention to provide a local oscillator generation circuit offering a greater flexibility for choosing the frequency range of the voltage controlled oscillator than currently existing implementations.
To this end, a LO frequency generation circuit of the invention generates an output LO frequency signal from an input frequency signal. The circuit comprises a switch circuit including a switch and a plurality of filters. The switch circuit is configured to filter the output LO frequency signal through one of the filters to produce a filtered frequency signal. A frequency divider is configured to multiply a frequency of the filtered frequency signal by a preset ratio for generating a secondary output frequency signal. The output LO frequency signal is generated in a mixer mixing the input frequency signal with the secondary output frequency signal.
A circuit of the invention enables to generate an output signal of a desired frequency by switching to the appropriate filter of the switch circuit. Thus, by setting the appropriate frequency of the input frequency signal and the appropriate characteristics for the filter and the divider, the desired output frequency signal may be generated. The desired frequency band may thereafter be obtained based on the frequency band of the input frequency signal. The input frequency signal may be obtained from a local voltage controlled oscillator (VCO). The characteristics of the switch circuit and the frequency divider may be determined based on the oscillating frequency of the VCO and the desired frequency of the output frequency signal to achieve. For each filter of the switch circuit a given output LO frequency may be generated. In an embodiment, the switch circuit allows switching between a low-pass filter and a high pass filter and the invention thereby enables deriving two frequencies and corresponding two frequency bands. An advantage of the invention is therefore to enable the generation of different frequency bands using a single voltage controlled oscillator. Another advantage of the invention is to enable the generation of frequency bands that are situated above and below the input frequency of the VCO. Thus, the invention enables a greater flexibility in the choice of the frequency bands at which the VCO operates. The frequency may be therefore chosen to optimize the phase noise and the tuning range and consequently, the frequency band of the VCO may be made smaller to reduce the phase noise incurred.
In another embodiment of the invention, the circuit of the invention may be used to generate other frequencies by inserting a circuit of the invention in lieu of the frequency divider. Such a circuit therefore comprises two switch circuits and in an embodiment where the switch circuit enables switching between a high pass filter and a low-pass filter, four frequencies may be generated from the four possible combinations of filters as will be shown hereinafter. In further embodiments, the output LO frequency signal may be generated from a cascade of circuits of the invention.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.